The present invention relates to rotatable valve assemblies and more particularly, but not by way of limitation, to a rotatable valve assembly which may be used as a pressure relieving device.
The use of various sorts of rupture disks and pressure relief valves to prevent overpressure of a fluid pressure containment structure are known. For example, U.S. Pat. No. 3,472,284 (Hosek) discloses a pressure seal in which disks 26, 32 rotate about offset shaft 24. A diaphragm 14 is clamped between the disks and housing to seal the assembly and the diaphragm is sheared by the rotation of the disks when a desired fluid pressure is present. In order to reseal the valve, the housing members 10, 12 must be opened and the valve taken out of service.
U.S. Pat. No. 3,039,482 (Goldberg) discloses a butterfly-type valve in which the longitudinal axis of the inlet and the longitudinal axis of the outlet are offset so that when the valve is forcibly oscillated and the hermetic sealing sheet 13 is sheared, the sealing ring 18 on the lower edge of the valve 16 will safely clear the rough and sharp sheared edge of sheet 13. The inlet and outlet body sections 11, 12 must be taken out of service and opened in order to reseal the valve after a pressure relieving event.
U.S. Pat. No. 2,304,491 (Allen) and U.S. Pat. No. 3,603,333 (Anderson) disclose traditional right-angle relief valves in which a shear pin is used to hold the valve in a closed, sealed position until a preselected pressure is experienced in the inlet to the valve. Upon overpressure, the valve shears the shear pin and is moved to an open, pressure relieving position. Since both of these valves use a shear pin, the tolerances between the member holding the shear pin stationary and the moving member which shears the shear pin are critical to provide a predictable relief pressure. For example, referring to the Allen patent, the diameter of the stem 8 and the internal diameter of the bushing 9 through which the pin 13 passes must be carefully matched to achieve an accurately predictable shear pressure. As the space or gap between the sides of the stem 8 and the internal diameter of the bushing 9 increases, the force required to shear the pin 13 diminishes rapidly. This requires precision machining and matching of the componentry that is difficult and expensive to repeatably produce and to maintain. Other shortcomings of the right-angle relief valves include the increased body size and the flow restriction created by the right angle turn in the flow passageway, as well as the tendency of the valve to flutter or chatter on the seat during pressure fluctuations because the valve must resist the entire fluid pressure exerted at the inlet, i.e., the shear pin and spring must directly resist the full force exerted on the face of the valve by the inlet pressure and directly absorb all pressure fluctuations.
U.S. Pat. Nos. 4,724,857, 4,787,409, 4,930,536, 4,977,918, 5,012,834, 5,067,511, 5,116,089, 5,146,942, 5,209,253, 5,226,442, 5,273,065, 5,297,575, 5,311,898, 5,318,060, 5,348,039, 5,373,864, and 5,433,239, all issued to Taylor, disclose pressure relief valves, emergency shutoff valves, vacuum/pressure release valves, and shutoff valves, which use a rupture or buckling pin which directly resists the substantially axial motion of a valve. The Taylor patents do not disclose the use of a rupture pin with a rotatable valve assembly, such as a butterfly valve, ball valve, plug valve, or the like; the use of a rupture pin which is not directly aligned with the motion of the valve; the use of a release means which increases the leverage the valve has on the buckling pin; or the use of a linearizing means to align the force the valve exerts with the axis of the buckling pin.
It is known to use rupture disks, such as reverse buckling rupture disks, to replace right-angle relief valves. Rupture disks have a lower cost per unit of capacity when compared to right-angle relief valves. However, rupture disks must be taken out of service to restore their pressure containing capability after a pressure relieving rupture. This requires either a redundancy (the use of multiple rupture disks in parallel), shutting down the system, or risking exposing the system to overpressure while the rupture disk is being replaced.
It is known to use rotatable valve assemblies, such as butterfly valves, ball valves, plug valves, and the like, to control or relieve fluid pressure in fluid containment systems. However, the prior rotatable valve assemblies known to the applicants typically require that the valve disk, ball, or plug, wipe or drag across a high friction, tight fitting seal made of elastomer or Teflon.RTM.; or use a seal which makes a "face" contact of substantial surface area around the valve. Therefore, a relatively high torque is required to open the prior valve assemblies, and this torque increases with time while the valve is in a closed position.
For example, Watts Regulator Company manufactures a Series QF Quarterflex butterfly valve which discloses a pressure assisted seat design. The line pressure exerts an upward force on the seat which forces the seat against the valve disk "accordion" style. The sealing surface of the seal is substantially in diametrical alignment with the seal seat which connects the seat to the retainer, i.e., there is a double fold in the seal which substantially aligns the seal seat and the sealing surface such that the sealing surface is not axially displaced from the seal seat. The sealing surface has a face of substantial axial dimension which creates a face seal of substantial surface area around the circumference of the disk. Such a face seal exerts substantially greater friction, requiring substantially greater torque to open and close, than the point or edge-type seal of the present invention. Watts' valve also uses a double offset shaft to reduce seat wear and enhance sealing by providing a camming action that is disclosed as lifting the disk off the seat.
U.S. patent application Ser. No. 08/519,653, which is assigned to the assignee of the present application, and which is incorporated herein by reference for purposes of disclosure, discloses a rotatable valve assembly which includes a seal which folds or bends about its rotational axis as the valve rotates from the closed to the open position. The seal has an endless protuberance which extends into a notch formed between the valve and housing in the closed position of the valve. Although the folding seal is a great improvement over prior devices, it requires between eight and thirteen foot pounds of torque to move the protuberance into and out of the notch as the valve closes and opens. The improved seal of the present invention reduces the torque required to open and close the valve to between one and three foot pounds.
There is a need for a valve assembly which will provide fluid pressure relief at an accurately predictable relief pressure and which may be returned to its pressure containing state after a pressure relieving event without taking the assembly out of service. There is also a need for a rotatable valve assembly which will reduce the frictional forces which must be overcome to rotate the valve between the open and closed position and in which the torque required to initiate rotation of the valve does not increase as the valve is left in a closed position for extended periods of time.